Effect of Tribocharging on Powder Packing

Publication Reference: 
FRR-66-04
Author Last Name: 
Castellanos
Authors: 
Javier Perez Vaquero, Miguel Angel Sanchez Quintanilla, Antonio Castellaos Mata
Report Type: 
FRR
Research Area: 
Powder Flow
Publication Year: 
2016
Publication Month: 
12
Country: 
Spain

Executive summary.

This reports presents the results of a series of experiments aimed at studying the factors

affecting the amount of charge in dispersed and bulk powders, for how long the charge

remains in the bulk powder before dissipating into the environment and the effect of the

electric charge on the solid fraction of a bulk powder. The powders used in this study

cover a range of particle sizes from 3 μm to roughly 1000 μm, although not all powders

were used in all the experiments. In the first experiment presented in this report, particles

are dispersed in a gas stream and charge due to collisions with a tribocharger. It is found

that the main factor affecting the maximum amount of charge per particle is the particle

size, since the charge is limited by the electric field on the surface of the particles and for

the same charge on a particle, the electric field on its surface scales with the square of

the diameter. In practical applications the particles may not experience enough number

of collisions with solid surfaces to charge up to their maximum level, but the results

presented in this report indicate than, on equal conditions, it is still the particle size the

main parameter affecting the particle charge. In the second experiment we measure the

charge distribution of the particles that come out of the tribocharger. We have found that

the particle charge has a very wide distribution spanning both polarities. This finding

may be explained if the charge transfer from the tribocharger to the dispersed particles

causes a shift in a pre-existent charge distribution in the direction of the transferred

charge. In consequence, the particles in a neutral bulk powder may carry electric charge,

but on some of the particles the charge is positive and on the others is negative. In the

third experiment we have measured the charge in a bulk powder formed by sedimentation

of highly charged particles. We have found that while particle settles, the layer of bulk

powder formed losses its charge. We propose a model that qualitatively describes the

decay of the charge in the bulk powder based on the assumptions that the charge in

the bulk powder has some mobility and that charge is dissipated on the surfaces of the

bulk powder by neutralization with ions existing in the surrounding gas in order to keep

the electric field on the surface of the powder at a value equal or below the breakdown

field in the gas. The amount of charge in a bulk powder results from an equilibrium

between charge dissipation into the surroundings and the accretion of new charge from the

incoming particles and according to the model, depends on the charge on the particles that

sediment, the mass flow rate at which they arrive and the effective electrical conductivity

of the powder. The effective electrical conductivity can be estimated from the typical time

for charge dissipation, which is of the order of minutes, yielding a value of the effective

electrical conductivity of the bulk powder of the order of nS/m. In the fourth set-up

we measure directly the effective electrical conductivity of some powders as a function

of consolidation and ambient humidity. The effective conductivity is found to be in the

order of nS/m and it is highly dependent on humidity and to a lesser extend, on particle

size. The strong dependence on humidity, specially for smaller particle sizes, may explain

why the charge on bulk powders seem to be highly unpredictable in environments in

which humidity is not controlled. In the fifth and last set-up, we measure the poured and

tapped densities of charged and uncharged powders in order to determine if there is an

effect of electric charge on the solid fraction, but within the accuracy of our experiment,

we have found none.